This invention relates to a method of and apparatus for determining the path of a well bore under drilling conditions.
To facilitate the extraction of oil and gas from the Earth, well bores are drilled by rotating a drill bit attached to the end of a drilling assembly, commonly referred to as a xe2x80x98bottom hole assemblyxe2x80x99. The path of the well bore must be precisely controlled so as to reach the required xe2x80x98targetxe2x80x99, the underground reservoir containing the hydrocarbons to be extracted, as efficiently as possible. At the same time, it is essential to ensure that the path of a new well bore is maintained at a safe distance and avoids existing well bores in the same oil field. To achieve these objectives, it is necessary to control accurately the path of the well bore whilst it is being drilled. This can be achieved by various means using vector measurements of the Earth""s magnetic and gravity fields derived using magnetic and acceleration sensors respectively to determine the inclination, azimuth direction of the well and tool face angle, or alternatively, by using acceleration sensors and gyroscopes capable of sensing components of Earth""s rate in order to derive the direction of the well path. The vector measurements in combination with depth information, derived from the well pipe tally for instance, are used to provide a measure of the well path on a xe2x80x98continuousxe2x80x99 basis throughout the drilling process.
U.S. Pat. No. 4,812,977 discloses a so-called strapdown inertial navigation system. The device utilises gyroscopes and accelerometers together with the necessary sensor drive electronics and signal processing capability. The system is capable of providing measurements of the orientation and/or position of the inertial system as the drilling process proceeds. These data define the instantaneous inclination and azimuth direction of the well path with respect to an Earth fixed coordinate frame of reference and/or the coordinate position of the device within the well bore with respect to the designated reference frame; this is usually defined in terms of the north, east and vertical position, or in polar coordinates as latitude, departure and depth. The inertial sensors are fixed rigidly to a support unit commonly and herein referred to as a platform. The platform may in turn be attached to the drill string assembly rigidly or via anti-vibration mounts.
The device which is the subject of this patent application seeks to extend the use of strapdown technology to facilitate its application for a broader range of well drilling applications; in particular, but not exclusively, to allow a strapdown inertial navigation system to be used to provide meaningful survey data whilst implementing the drilling process known as rotary drilling, in which the drill bit is driven from the surface causing the complete tool string to rotate at the required drill speed in order for the rotary motion to be transmitted to the drill bit at the bottom of the well. In the event that a strapdown inertial system were to be used in such an application, the drill string rotation rate may well exceed the measurement range of the gyroscope and the gyroscope scale factor error would give rise to an unacceptably large measurement offset during a high speed drilling operation.
According to a first aspect of the invention there is provided apparatus for determining the path of a well bore during drilling, comprising an inertial measurement unit for providing data representative of position, velocity and attitude, the measurement unit comprising a plurality of inertial sensors mounted on a platform assembly which is, in use, disposed within a drill string, and a drive unit for rotating the platform assembly so as to control the rate of angular displacement of the platform assembly with respect to an Earth fixed reference frame.
The term xe2x80x9cEarth fixed reference framexe2x80x9d typically means a Cartesian co-ordinate frame the axes of which are coincident with the directions of true north, east and the local gravity vector.
Preferably, the inertial sensors comprise accelerometers and gyroscopes and the inertial measurement unit further includes means for integrating output signals of the accelerometers once to provide information representative of velocity and twice to provide information representative of position, and means responsive to output signals of the gyroscopes for resolving the accelerometer outputs into an Earth fixed reference frame and to generate estimates of inclination azimuth and tool face angles.
Other preferred and/or optional features of the first aspect of the invention are set forth in claims 3 to 6.
According to a second aspect of the invention there is provided a method of determining the path of a well bore during rotary drilling using the apparatus according to the first aspect and rotating the platform assembly to cause the platform assembly to remain stationary, or near stationary, in angular terms with respect to an Earth fixed reference frame.
According to a third aspect of the invention there is provided a method of determining the path of a well bore during rotary or mud motor drilling using the apparatus according to the first aspect and rotating the platform assembly at a fixed angular rate with respect to an Earth fixed reference frame.
According to a fourth aspect of the invention, there is provided a method of using apparatus according to the first aspect of the invention, wherein the platform assembly is rotated by the drive unit at a slow angular rate relative to an Earth fixed reference frame to cancel out the effects of residual bias errors in the gyroscopes.
According to a fifth aspect of the invention, there is provided a method of using apparatus according to the first aspect of the invention, wherein the drive unit is used to decouple and maintain control of rotation of the platform assembly relative to tool string rotation to reduce the effects of scale factor errors in the gyroscopes.
The invention is particularly applicable to rotary drilling, but the system described herein could also be used to provide well trajectory data when operated during the drilling process known as mud-motor drilling. In this case, the drill bit is driven by the circulation of drilling fluid or xe2x80x98mudxe2x80x99 which is pumped from surface down the drill pipe to the motor at the well, before returning to the surface via the annulus formed between the drill pipe and the wall of the well bore. Energy is imparted to the drill bit via an impeller or mono device causing the drill bit to rotate. In this method of drilling, the drill string rotation remains nominally at zero throughout the process. However, there are still benefits to be obtained in terms of system accuracy and ruggedness through installing the inertial measurement unit on a stable platform assembly as described above.